The oldest known human ancestor footprints, dated to 3.7 million years ago, reveal that some of the earliest members of our family tree walked fully upright with feet similar to ours, according to new research.

The findings, published in the Journal of the Royal Society Interface, push back the date for upright walking in our ancestry by nearly 2 million years. That's because previous studies had concluded this trademark gait emerged in the genus Homo about 1.9 million years ago.

The 3.7 million-year-old footprint maker, likely a species called Australopithecus afarensis (of the "Lucy" fossil fame), walked in an even less ape-ish way than some humans do today.

Crompton, a professor in the University of Liverpool's School of Biomedical Sciences, explained that many researchers believed loss of the "so-called mid-tarsal break," a flexing of the side midfoot, "distinguished humans from non-human apes." This flexing can contact the ground and leave behind a mark in footprints. But Crompton and his colleagues found that certain people today create such footprints, while some of the ancient prints were lacking the distinguishing feature.

The researchers made this determination after studying the ancient footprints, found at a site called Laetoli in Tanzania. At least one, or possibly two, individuals left behind the footprints as they walked through damp volcanic ash.

Detailed imaging, based on methods employed in functional brain imaging, yielded clear, three-dimensional views of the 11 intact prints in the Laetoli trail. The scientists then compared these ancient footprints with prints made by modern humans and other living great apes.

It's now known that the prehistoric walker moved at just over 3 1/2 feet per second.

"Walking was completely erect," Crompton said. "Push-off was substantially by the big toe and a medial arch was present. There was no collapse of the lateral side of the foot, as has been recently suggested."

The probable walker, A. afarensis, is best known via a partial skeleton named "Lucy." Lucy and other members of her species are thought to have lived at least some of their time in trees. Since Lucy was already walking upright in a very human way, the new study strengthens the theory that erect, two-footed walking evolved in a tree-living ancestor of living great apes and humans.

Crompton explained that this way of moving was "probably faster than quadrupedalism on the ground in an animal built like great apes are." It also may have "enhanced feeding from low branches or bushes."

Upright walking on two feet additionally could have helped with visual displays and showing aggression, he said. All of this may have happened as forest canopies broke up, forcing A. afarensis to leave its tree homes for a more terrestrial way of life.

Unlike modern humans, however, this species had short legs and a long body. When compared to other animals, we have just the opposite: relatively long legs and a short body.

Sarah Elton of the Hull York Medical School told Discovery News, "Although there have been heated disagreements over how to interpret the Laetoli footprints, this incredibly detailed and well thought-out study must surely help to put some of the debate to rest."

She continued, "I was surprised by such clear evidence for a very 'human-like' foot morphology and walking style, particularly the presence of a longitudinal arch, and 'toe off' when walking."

Elton added, "This strongly suggests that the makers of the footprints, although living over 3 million years ago, shared many features of their locomotion with us."

Crompton and his team next hope to determine when our ancestors first walked, or ran, over very long distances, enabling them to colonize the world.